The present invention relates to countermeasures for transmission errors in a speech coding transmission system.
Analog or digital data transmission has long used wire telecommunication networks, in which errors during transmission are very rare, and it has been known that the error rate is in the range of 10.sup.-4 to 10.sup.-8.
With such a technical background, speech coders and speech decoders have been developed for application to a digital data transmission system via a wire telecommunication network. For example, a speech coder or decoder of an adaptive differential PCM (ADPCM) system has been developed on the assumption of an error rate less than 10.sup.-4.
The wire telecommunication network is now partly being substituted by radio transmission for a cordless telephone system, for example. From the viewpoint of matching with the wire telecommunication network, it is appropriate, in this instance, to use a conventional speech coder or decoder such as the ADPCM system.
In radio transmission, however, occurrence of a transmission is inevitable. If the real-time transmission in the radio transmission system is ignored, the transmission errors could be reduced to some extent by repeating or like technique, but the repeating technique cannot be used in speech signal processing as of conversation voice signal because the real-time property (a delay time in the range of 10 to 20 ms) is requisite to such signal processing.
Accordingly, it is necessary to take some countermeasures for errors in the application of the speech coder or decoder to the system wherein the wire telecommunication network partly includes a radio section. One possible method is the use of an error correcting code, but this method has a defect of an increase in the quantity of information to be transmitted. Therefore, if an appropriate countermeasure can be made by the speech coder or decoder itself against errors, its utility value will be great. However, such a technique is unknown, because the speech coder or decoder intended primarily for use in the wire telecommunication network as mentioned above has not been applied to the radio telecommunication network in the past.